Read the first two lines of "Yet Do I Marvel" out loud (we did). What do you hear? A lot of T and D sounds, right? Let's be more specific: "doubt", "not", "God", "good", "kind", "And", "did", "stoop", "to", "could", and "tell". This is called consonance, or the repetition of the same consonant sound. Sure, T and D aren't exactly the same, but our mouths make the same shape when we say those letters. It's almost like a rhyme of sorts, but not quite.
Great, but what's the point? Why would Countee Cullen want so many similar sounds so close together? One reason may be because his name is a great example of alliteration, but more likely it's because he's creating a steady rhythm in his lines. The thud of that T and that D link the words together like a sonic chain, and they also mimic a sense of certainty. It's almost like those sounds are little walls holding up the words. There's a confidence there, like the sound of someone marching in boots. In this way, the sound of the words mimics the meaning; at the beginning of the poem, the speaker is declaring his certainty about God's goodness. Not only the meaning of the words, but the sounds of the words, get this message across.
There are several other examples of consonance in this poem. Can you find them? Remember the letters of the word don't have to be the same to have the same sound (for example: "quibble" and "could" both have that hard "c" sound). How do they create a rhythm in the poem, and do their sounds mirror the poem's message?
But that's not all. Cullen actually employs several different sonic techniques in this poem. Look at the first half of the first line again. "I doubt not God is good". Looks like there is a lot of O going on, no? What's fascinating is that not only is there consonance at play here, but assonance as well. Assonance is the repetition of vowel sounds. The "ou" in "doubt," the "o" in "not" and "God," and the double-O in "good" sound similar, right? That helps link the words together and reinforce their message. The speaker is saying he doesn't doubt God's goodness, right? Well, those words, "doubt," "not," "God" and "good" all sound alike, so again, their sound relationship emphasizes the speaker's message.
Countee Cullen does this throughout the poem. Shmoop doesn't want to have all the fun ourselves, so go through and look for repeated consonant and vowel sounds. Your ear already picked them up while you read, but do it again and you'll be surprised how often they pop up in this poem, and try to suss out how they enhance the poem's meaning as you find them.